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Topic: Basic Rocket Science Q & A (Read 281649 times)

You could go faster than what Jim said if you could apply continuous delta-V directed radially inward. For example, if you had a hypothetical aircraft that flew in the upper atmosphere upside down generating lift toward the center of the Earth instead of away from it like a usual airplane, you could in theory go faster. Practice and theory in this regard are still quite a ways apart.

Let's say you're in a perfectly circular orbit around a gravitation point source (no gravitational gradients or anomalies). Let's move our frame of reference into your frame and call your now-fixed location "A". If you were to apply a small delta-V in either direction along the velocity vector, what would be the shape of your flight path relative to point "A"?

Interestingly enough, you don't find many natural orbital systems faster than a few hours. The closest binary stars have periods of greater than ~3 hours, as do the closest asteroid satellites...

Although slightly OT, a quick search in the ADS database reveals that some exotic star systems manage an even faster pace. For instance, Stella et al. claimed the discovery of a so called low mass X-ray binary with an 685 s orbital period.

So, is it easy to vary the thrust force in chemical rockets? How does that change the propellant flow rate? If I halve the thrust, does the propellant flow rate halve too?

No. You have to look at combustion stability and turbodynamics. The motor may not be able to operate at that point without tearing itself up. Even if those two items are still healthy, you have to do the detailed aerothermal analyses to see how the nozzle flow works to determine what mass flow effects half thrust. It would probably be within a few percent of half the mass flow, assuming the nozzle was still choked.

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If I like something on NSF, it's probably because I know it to be accurate. Every once in a while, it's just something I agree with. Facts generally receive the former.

I have heard that there are low energy but slow orbital transfers for example to get from the earth to the moon, or to get back. I looked on line but mainly found papers with very scary titles. I could not find a good summary of what savings in fuel are plausible, at what cost in increased travel time. Does anyone know of such a summary or just some interesting specific examples? Im just trying to get an idea of whether taking 6 months saves 25% of your propellant or is an order of magnitude improvement etc.

Google interplanetary superhighway and/or low energy transfers. That'll get you fairly layman, or at least technical generalist, answers. Here's a Discover article that says Lunar Observer could have saved 25% of its fuel and 30% for a generic mission.

Google interplanetary superhighway and/or low energy transfers. That'll get you fairly layman, or at least technical generalist, answers. Here's a Discover article that says Lunar Observer could have saved 25% of its fuel and 30% for a generic mission.

Thanks for that link.It occurs to me that perhaps whatever I read initially was also assuming some more efficient but lower thrust form of propulsion that becomes practical if the timeframe is six months?

What exactly will happen when a thrust, that is perpendicular to the orbital plane, is applied to an orbiting spacecraft? It would no doubt change the inclination of the orbital plane, but is that all that's gonna happen? Will it somehow affect the altitude?

The application of thrust perpendicular to the orbital plane will increase the speed, although if the magnitude of the burn is small compared to orbital speed (about 7800 m/s in low earth orbit), then this effect will be very small. It's just the Pythagorean theorem. If the initial orbit is circular, then the new orbit will be in a different plane, as you surmise, and elliptical, with perigee at the point where the burn was performed.